The proposed study is a continuation of research activities directed towards understanding the mechanism controlling the movement of potassium across epithelial membranes. Specific goals include both an elucidation of the processes which regulate the transfer of potassium ions as a general phenomena of membrane physiology and a delineation of these processes as they specifically relate to the absorption and secretion of potassium along various segments of the intestinal tract. The experimental design will include studies to determine the effect of several physiological variables, including changes in dietary potassium, alterations in acid-base balance and mineralocorticoids on the direction and magnitude of potassium movement in small bowel and colon. These experiments will utilize in vivo perfusion of intestinal segments to estimate net movement of water and electrolytes, transmural PD and the determination of the specific activity of Na-K-ATPase in mucosal cells. It is anticipated that these studies will provide several models, characterized by markedly different rates and directions of potassium movement, in which an analysis of the factors controlling potassium movement can be performed. In the second portion of the proposal, segments of intestinal tissue will be examined in vitro to determine the relative importance of electrochemical properties and permeability characteristics of mucosal cells on overall transepithelial potassium movements.